Traditional IT suppliers, including EMC, Cisco, and NetApp, are under siege from cloud operators like Amazon and Google. Pricey start-ups abound. A second chance for VMware.

The cloud changes everything. The technology leaders of the past, including Hewlett-Packard , Cisco Systems , and EMC ,face unprecedented threats as computing moves out of corporate data centers and onto the cloud that companies like Google , Amazon.com , and others have created.

Over the past decade, these large Web companies have figured out how to build more-efficient computing facilities than the information-technology teams within large companies could ever imagine. Now these firms are renting their vast computing...